Atividade microbiana do solo e produtividade do algodoeiro em resposta às plantas de cobertura e adubação potássica e nitrogenada

Abstract

Soil biologic activity is influenced by quality and quantity of organic matter, however it is not clear management of nitrogen (N) and potassium (K) fertilization interferes soil biologic activity. These nutrients are highly required by cotton, assist in vegetative growth, influenced in increase in yield and fiber quality.The objective was to evaluate the microbial activity of the soil, yield and quality of the cotton fiber grown in straw black mucuna and Urochloa ruziziensis with or without inoculation under different times of application of K and doses of N. The experiment was conducted in an Argisol soil Dystropheric red with sandy texture, using the randomized blocks design with five replications in a split-split plot scheme. In the plots cover crops were allocated: 1-Mucuna (M); 2- Fallow (F); 3- Ruzigrass+Azospirillum (R+A); 4- Ruzigrass+Mucuna; 5- Ruzigrass+Azospirillum+Mucuna (R+A+M) e 6- Ruzigrass; in the subplots the management of K: 140 kg ha-1 applied in pre-planting in cover crops and/or post-planting in cotton (cover). Two experiments were carried out, one at the dose of 80 and the other at 120 kg ha-1 of N. The microbial biomass carbon showed a higher content in the presence of mucuna and single forage; already microbial biomass nitrogen in the association of ruzizizensis + Azospirillum + mucuna and in single ruziziensis, with a dose of 120 kg ha-1 of N and K applied in pre-planting. Enzymes such as dehydrogenase and fluorescein diacetate hydrolysis (FDA) were influenced only by cover crops at a dose of 80 kg ha-1 of N. However, urease and β-glucosidase expressed higher levels at a dose of 120 kg ha-1 of N with the supply of K in pre-planting and under straw mucuna+ruziziensis. The cover crops increased the N content in the soil, with the single mucuna being the treatment that showed the highest content of inorganic N in both doses of N. There was an increase in the cotton yield sown under straw of inoculated ruziziensis+mucuna, with 120 kg ha-1 of N and K in post-planting, but with no effect on fiber quality. As the cover crops associated with ruziziensis + Azospirillum + mucuna provide better results under microbial and enzymatic activity with 80 kg ha-1 of N and K in post-planting; already tried a cotton fiber that expressed higher yields with K applied in pre-planting and 120 kg ha-1 of N. However, the fiber quality did not express an effect under the management employed.